(a) Field of the Invention
The present invention relates to a wavelength tunable light source, and more particularly, to a wavelength tunable light source capable of tuning a rapid wavelength and improving stability of an output wavelength.
(b) Description of the Related Art
Research and development on a wavelength division multiplexing (WDM) based passive optical network (PON) for providing voice, data, and broadcast converging service that will be largely activated within several years are in progress all over the world. Hereinafter, the wavelength division multiplexing based passive optical network will be referred to as ‘WDM-PON’.
The WDM-PON is a scheme in which communication between a center office (CO) and a subscriber is made by using wavelengths allocated to subscribers, respectively. Since an unique wavelength is used for each subscriber, security is excellent, a larger dedicated bandwidth is available irrespective of protocols.
However, since the WDM-PON is a technology that transmits a signal to each subscriber by multiplexing various wavelengths to a single optical fiber by using a WDM technology, the WDM-PON needs different light sources as many as the number of subscribers.lmplementation, installation, and management of the light source for each wavelength operate as a large economical burden to both a user and a network provider are blocking compatibilization of the WDM-PON. In order to solve this problem, wavelength tunable light source capable of selectively change the outputwavelength of a light source is being actively researched.
A typical example of the wavelength tunable light source may include a planar lightwave circuit (PLC) based-external cavity laser (PLC-ECL) in which an optical lens is configured by passively or actively aligning individual optical elements such as a semiconductor laser diode or a reflective semiconductor optical amplifier, a planar lightwave circuit (PLC), and an optical fiber. Hereinafter, the planar lightwave circuit based external cavity laser is referred to as ‘PLC-ECL’.
FIG. 8A is a block diagram for each function of a well known PLC-ECL type wavelength tunable light source. Referring to FIG. 8A, the well known PLC-ECL wavelength tunable light source 10 includes a gain region 11, a wavelength tunable region 12, a phase control region 13, and an optical fiber 14.
A reflective semiconductor optical amplifier (RSOA) or a semiconductor laser diode (LD) is used as the gain region 11. A front facet of the RSOA or the semiconductor LD is coated with an anti-reflection layer and a rear facet of the RSOA or the semiconductor LD is coated with a high reflective layer in order to form a resonator. Hereinafter, the RSOA will be used as the gain region 11 for convenience.
The wavelength tunable region 12 is formed by a PLC element and the PLC element includes a WAVEGUIDE, in which a Bragg grating is formed in a partial section of the core region of the waveguide and a thin-film metal heater is disposed adjacent to the diffraction grating. When the RSOA as the gain region 11 and the core layer of the PLC element with the Bragg grating are optically coupled with each other through an active or passive alignment method and thereafter, driving current is applied to the RSOA, an external resonator is formed between the high reflective layer of the rear facet of the RSOA and the Bragg grating in the PLC element and a optical wave having a specific wavelength that coincides with an effective period of the diffraction grating oscillates. Additionally, when an optical output of the PLC element is combined with the optical fiber 14 such as a single-mode optical fiber, etc., it becomes a light source.
In the case of the PLC-ECL wavelength tunable light source 10, the PLC-type lightwave is manufactured by using an organic compound (polymer) having a relatively higher thermooptic coefficient for tuning an resonant wavelength and when current is injected through electrodes at both ends of the thin-film metal heater, heat generated from the thin-film metal heater increases the temperature of the waveguide core layer adjacent thereto. As a result, a refractive index of the waveguide core layer is changed by a thermooptic effect, causing the change of refractive index of the Bragg grating to be shortened so as to tune the emitted optical wavelength of the PLC-ECL.
The thin-film metal heater, both electrodes, and the Bragg-grating is comprised of the wavelength tunable region 12 and the thin-film metal heater and both electrodes that are disposed in parts of the waveguide core layer. The phase control region 13 controls a round trip phase of the emitted light from the PLC-ECL
The PLC-ECL based wavelength tunable light source 10 has a simple structure to be easily implemented and has a small number of control parameters to be easily operated and in addition, it can separate key function blocks at the same time and verify them to rapidly and easily correct a defect when individual components have errors. Further, the PLC-ECL based wavelength tunable light source 10 can be implemented costeffectively.
However, since the PLC-ECL based wavelength tunable light source 10 uses the thermooptic effect for tuning the wavelength, a power consumption for this is relatively larger and when a polymer waveguide is used as a wavelength tunable filter, a heat transfer characteristic of the polymer itself is not good for varying ambient temperature, thus, the stability of a tunable wavelength is severely degraded. In addition, as the wavelength is tuned repetitively, it is difficult to tune the wavelength to be a desired value due to a high fatigue degree characteristic of the polymer material itself.
In addition, since sizes of components constituting the wavelength tunable light source are relatively larger, it is difficult to manufacture a small-sized optical transceiver. A packaging process including optical coupling or component alignment is more complicated than other wavelength tunable light sources, thus, a manufacturing cost may be increased. In addition, slope efficiency of the light source itself is lower than the that of a wavelength tunable light source integrated on a single substrate due to optical coupling.
FIG. 8B shows a known sampled grating distributed Bragg reflector (SG-DBR) laser diode complementing the disadvantages of the above-mentioned PLC-ECL based wavelength tunable light source 10. The well known SG-DBR laser diode 20 includes two SG-DBR regions 21 and 24, a gain region 22, and a phase control region 23. In addition, the SG-DBR laser diode 20 further needs external control circuits such as a Vernier control circuit 27 for continuous wavelength tuning, an offset control circuit 28 for discrete wavelength tuning, a phase control circuit 26 of the phase region and a gain control circuit 25 in order to tune the wavelength of SG-DBR output.
Referring to FIG. 8B, when current is applied to the gain region 22, an optical wave distributed throughout a wide wavelength is generated by self emission. In the case of the optical wave, only an optical wave having a predetermined wavelength can be resonated in the laser diode by SG-DBR regions at both ends to cause the laser diode to oscillate at the wavelength.
A sampled grating structure is formed in the SG-DBR regions 21 and 24. Only the optical wave corresponding to the predetermined wavelength is reflected by the sampled grating. A wavelength of a center peak on a reflection spectrum is a Bragg wavelength determined by a diffraction grating period and an interval between wavelengths each having a peak is determined by period of sampled gratings. That is, SG-DBR regions of sampled gratings having different periods are integrated on both terminals of the laser diode, such that the laser diode has oscillationb characteristics at a predetermined wavelength having matching peaks among peaks of the reflection spectrum of the SG-DBR region.
In addition, when a refractive index of the SG-DBR region is changed by current, etc., each peak of the reflection spectrum moves while maintaining an interval between the wavelengths. Wavelengths of the matching reflection peaks are changed due to movement of the reflection peak so as to tune the oscillation wavelength.
The phase control region 23 serves to maximize the power of the oscillating wavelength by matching a longitudinal mode with continuous wavelength tuning or the reflection peak by adjusting an interval between longitudinal modes of the gain region 22 generated by the SG-DBR region. According to such a principle, continuous/discrete wavelength tuning is possible by properly adjusting refractive indices of the SG-DBR regions 21 and 24 at both ends and the phase control region 23 by using current.
However, the SG-DBR laser diode has a structural limit in which since the refractive indices of the SG-DBR regions at both ends and the refractive index of the phase region should be changed for tuning the wavelength, an external circuit for controlling the elements is complicated and optical coupling efficiency is decreased due to a loss generated in the SG-DBR regions integrated on both ends for tuning the wavelength.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.